KrF excimer lasers are the state of the art light source for integrated circuit lithography. Such lasers are described in U.S. Pat. No. 4,959,840, U.S. Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F2 lasers which operate at about 157 nm. These lasers, the KrF laser, the ArF laser and the F2 lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F2 laser merely by changing the gas concentration, increasing the discharge voltage and modifying the controls and instrumentation to accommodate the slightly different wavelength.
A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIGS. 1, 1A and 1B. A cross section of the laser chamber of this prior art laser is shown in FIG. 1B. As shown in FIG. 1A, pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 to 2000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 to 2,000 cm per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18 (sometimes referred to as a line narrowing package or LNP). Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
High voltage power supply module,
High voltage compression head module,
Commutator module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 1B. Flow is counter-clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor bank comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, one or two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors 19 reaches about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 to 2 cm per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one half to one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy. These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance.
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band xe2x80x9cseedxe2x80x9d beam into a gain medium. In one such system, a laser called the xe2x80x9cseed laserxe2x80x9d or xe2x80x9cmaster oscillatorxe2x80x9d is designed to provide a very narrow laser band beam and that laser beam is used as a seed beam in a second laser. If the second laser functions as a power amplifier, the system is typically referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is usually referred to as an injection seeded oscillator (ISO) and the seed laser is usually called the master oscillator and the downstream laser is usually called the power oscillator.
In gas discharge lasers of the type referred to above, the duration of the electric discharge is very short duration, typically about 20 to 50 ns (20 to 50 billions of a second). Furthermore, the population inversion created by the discharge is very very rapidly depleted so that the population inversion effectively exists only during the discharge. In these two laser systems, the population in the downstream laser must be inverted when the beam from the upstream laser reaches the second laser. Therefore, the discharges of the two lasers must be appropriately synchronized for proper operation of the laser system. This can be a problem because within typical pulse power systems there are several potential causes of variation in the timing of the discharges. Two of the most important sources of timing variations are voltage variations and temperature variations in saturable inductors used in the pulse power circuits. It is known to monitor the pulse power charging voltage and inductor temperatures and to utilize the data from the measurements and a delay circuit to normalize timing of the discharge to desired values. One prior art example is described in U.S. Pat. No. 6,016,325 which is incorporated herein by reference. There in the prior art timing errors can be reduced but they could not be eliminated. These errors that ultimately result are referred to as xe2x80x9cjitterxe2x80x9d.
A typical KrF laser has a natural bandwidth of about 300 pm (FWHM) centered at about 248 nm and for lithography use, it is typically line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 500 centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easily tuned over a large portion of their natural bandwidth using the line narrowing module 18 shown in FIG. 2. Also for the KrF and ArF lasers, the absolute wavelength of the output beam can be determined accurately by comparing its spectrum to atomic reference lines during laser operation. F2 lasers typically produce laser beams with most of its energy in two narrow lines centered at about 157.63 nm and 157.52 nm.
Often, the less intense of these two lines (i.e., the 157.52 nm line) is suppressed and the laser is forced to operate at the 157.63 nm line. The natural bandwidth of the 157.63 nm line is pressure dependant and varies from about 0.6 to 1.2 pm. An F2 laser with a bandwidth in this range can be used with lithography devices utilizing a catadiophic lens design utilizing both refractive and reflective optical elements, but for an all-refractive lens design the laser beam should have a bandwidth to produce desired results. It is also known that the centerline wavelength of the output beam will vary somewhat depending on condition in the discharge region.
Lasers for lithography equipment are very complicated and expensive. Further reduction in bandwidth could greatly simplify the lens design for lithography equipment and/or lead to improved quality of integrated circuits produced by the equipment. Thus, a need exists for lithography lasers (including KrF, ArF and F2 lasers) with substantially reduced bandwidth.
The wavelength of KrF and ArF lasers is relatively easily controlled over ranges of a few hundred picometers corresponding to their natural bandwidths. The F2 laser or the other hand has in part been considered untunable since the a large portion of its output is concentrated in two narrow lines. Several techniques have been prepared for selecting one of the lines and eliminating energy in the other line.
In many line selected laser systems, such as the KrF and ArF systems, a buildup in a narrow frequency band suppresses buildings in other spectral regions. However, weak interaction between the various laser frequencies may reduce the effect of this mechanism. F2 laser systems provide very large gains allowing substantial intensity build-up or merely a single pass through the gain medium. Initial light levels make originals as so-called spontaneous emissions at any potential laser line. When these spontaneous emissions are amplified, the light is referred to as amplified spontaneous emissions (ASE) and it may become a part of the output laser beam reducing the quality of the beam. In the case of the F2 laser many efforts have been made to suppress the 157.52 nm line while maintaining efficient production of the desired 157.63 nm line. For use as a lithography light source, there is a desire to reduce the intensity of the 157.52 nm line to less than 01.% of the 157.63 nm line.
What is needed is a better F2 laser system in which the 157.52 nm line is suppressed to insignificance.
The present invention provides a narrow band F2 laser system having two laser subsystems. The first laser subsystem is configured to provide a very narrow band pulsed beam at a first narrow wavelength range corresponding to a first natural emission line of the F2 laser system. This beam is injected into the gain medium of the second laser subsystem in a first direction where the beam is amplified to produce a narrow band pulsed output beam. The seed laser subsystem also produces a second pulsed beam at a second wavelength range corresponding to a second natural emission line of the F2 laser. This line is injected into the gain medium of the second laser subsystem in a second direction opposite said first direction. The second beam is amplified in the gain medium of the second laser subsystem depleting the gain medium of gain potential at the second wavelength range. (This amplified second beam is preferably wasted.) With the gain potential at the second undesired wavelength the range thus reduced the portion of light at the second wavelength range in the output beam is greatly reduced. In a preferred embodiment a pulse power supply is provided which is specially configured to precisely time the discharges in the two laser subsystem so that the discharges are properly synchronized, and laser gas comprises F2 at a partial pressure less than about 1% with a buffer gas comprised of helium or neon or a combination of helium and neon. Control of center wavelength of the output beam may be provided by adjusting one or more of the following parameters in the first laser: the total laser gas pressure, the relative concentration of helium or neon, F2 partial pressure, laser gas temperature, discharge voltage and pulse energy.
For precise jitter control in preferred embodiments include a pulse power system with a pulse transformer unit having two sets of transformer cores. A single upstream pulse compression circuit provides high voltage pulses in parallel to the primary windings of all of the cores in both sets. Separate secondary conductors (one passing through one set of cores and the other passing through the other set of cores) provide very high voltage pulses respectively to separate downstream circuits supplying discharge pulses to the electrodes in each of two separate laser chambers. In preferred embodiments line narrowing is accomplished within the resonant cavity of the seed laser and/or the output of the seed laser could be line narrowed using a pre-gain filter.